A modern modem 100, as illustrated in FIG. 1, typically includes a digital signal processor or microprocessor 102, a coder/decoder (“codec”) 132 for converting digital signals from the DSP 102 to an analog form capable of transmission over a telephone line and for converting analog signals from the telephone line to digital form, and high-voltage (“HV”) components 130 that interface with the telephone line. In order to isolate the DSP 102 from voltage fluctuations on the telephone line, the codec function is conventionally implemented via two circuits—a system-side interface circuit (“SSIC”) 106 and a line-side interface circuit (“LSIC”) 118, which communicate across an isolation barrier 117.
The SSIC 106 includes a system I/O interface 108 for communication with the DSP 102, a conventional sigma-delta modulator 112 for converting forward-going data signals to forward-going sigma-delta signals, a conventional integrator-based sigma-delta decoder circuit for decoding reverse-going sigma-delta signals into data signals, and an isolation barrier interface circuit 114 for transmitting and receiving sigma-delta signals to and from the LSIC 118 across the isolation barrier 117. The SSIC 106 may further include a protocol framing circuit 116, which functions to organize the data transmitted and received by the isolation barrier interface circuit 114, and a barrier clock controller 113 and associated voltage-controlled oscillator 115, which together form a variable-rate clock generator for generating the barrier clock signal.
The LSIC 118 includes an isolation barrier interface circuit 120, a line-side sigma-delta digital-to-analog converter (“DAC”) 126 whose output is connected to a transmit buffer 128, and a sigma-delta analog-to-digital converter (“ADC”) 122 whose input is connected to a receive buffer 124. The LSIC 118 may further include a conventional clock-and-data recovery circuit 125 to derive a local clock signal from the received signals from the isolation barrier. Each of isolation barrier interface circuits 114, 120 may be any suitable isolation barrier interface circuit for communication across an isolation barrier, such as that described in U.S. patent application Ser. Nos. 11/159,537 and 11/159,614 incorporated above.
Conventional modems typically also must accommodate a wide variety of communication rates. For example, a modem complying with the CCITT v.34 standard must be capable of communicating at a variable symbol rate (or baud rate) that may range from 2400 Hz-3429 Hz, as illustrated in Table 1 below.
TABLE 1Symbol rateSample rateΣΔ RateApplication[Hz][Hz][MHz]V.34240072001.8432AudioN/A80002.0480V.34274382282.1066V.34280084002.1504V.34300090002.3040V.34320096002.4576V.343429102872.6335Audio/N/A110252.8224optional
If the ADC sampling rate is selected to be factor of 3 times the symbol rate, the ADC 122 must have a sampling rate ranging from 7200 Hz-10,287 Hz (and as high as 11,025 Hz if the telephone signal is an analog audio signal rather than a digital modem signal). In addition, the sigma-delta (ΣΔ) rate is conventionally selected so that the analog signal is oversampled at a predetermined multiple (e.g., 256) times the sampling rate. As such, the sigma-delta ACD 122 must operate at a sigma-delta rate that ranges between 1.843 MHz and 2.822 MHz.
This wide range of the required sigma-delta rate (1.843 MHz-2.822 MHz) represents a design constraint on the barrier interface (the communication link formed by interface circuits 114 and 120 and isolation barrier 117). For successful full-duplex operation, during each EA sample interval, one forward ΣΔ sample and one reverse ΣΔ sample must be communicated across the isolation barrier between the SSIC 106 and the LSIC 118. In other words, the data rate of the barrier interface must be variable, depending on the sigma-delta rate.
The desired variable data rate for the barrier interface has conventionally been obtained by varying the barrier clock rate to obtain the desired data rate. In a simplified example, if the modem 100 establishes a v.34 communication with another modem at a symbol rate of 2,400 Hz (for which a ΣΔ rate of 1.843 MHz is needed), the DSP 102 or some other barrier clock controller 113 may set the barrier clock rate to a rate equal to two times 1.843 MHz, or 3.686 MHz, so that during each ΣΔ interval, at least one forward ΣΔ sample and one reverse ΣΔ sample may be transmitted across the barrier interface. In contrast, if the modem 100 establishes a v.34 communication at a symbol rate of 3,429 Hz (for which a ΣΔ rate of 2.634 MHz is needed, per Table 1), the barrier clock may be set to a rate of two times 2.634 MHz, or 5.268 MHz, again so that during each ΣΔ interval, at least one forward ΣΔ sample and one reverse ΣΔ sample may be transmitted across the barrier interface. Thus, the clock rate in this simplified example would have to be able to operate over the range from 3.686 MHz to 5.268 MHz (i.e., an increase of 42%) to accommodate the full range of v.34 symbol rates. Moreover, the barrier clock rate would have to be correspondingly increased if control and status information was to be communicated during each ΣΔ interval.
Unfortunately, this conventional technique of varying the barrier clock as a function of the symbol rate or sigma-delta rate causes at least two difficulties. First, if the LSIC 118 derives its local clock from the barrier signals via a clock recovery circuit, the clock recovery circuit loses synchronism with the barrier signals each time the barrier clock changes. Until the clock recovery circuit re-acquires the new clock rate, the SSIC 106 and the LSIC 118 are unable to communicate. Second, the clock generating circuit in the SSIC 106 and the clock recovery circuit in the LSIC 118 are relatively complicated and expensive, because they must accommodate the entire range of clock rates across the barrier.